The Sig steel base cartridge is ingenious since it allows for 80,000 PSi, but why not employ that concept in a Grendel-based cartridge (.22 ARC for example with an 80 grain pill or 6mm ARC with a 105 grain pill? You could motivate it at +3000 fps from a 16.5" barrel and still use the M4. With the 6mm ARC, 800-yard performance would be within an eyelash of the .227 Sig Fury but with much lighter ammo and rifle.
This!
